The present invention relates generally to the art of nuclear medicine. It finds particular application to nuclear imaging techniques and apparatuses employing emission and transmission tomography. Although the present invention is illustrated and described herein primarily in reference to positron emission tomography (PET) and single photon emission computed tomography (SPECT), it will be appreciated that the present invention is also amenable to other noninvasive investigation techniques and other diagnostic modes in which a subject or patient is examined with transmitted radiation.
Diagnostic nuclear imaging is used to study a radionuclide distribution in a subject. Typically, one or more radiopharmaceuticals or radioisotopes are injected into a subject. The radiopharmaceuticals are commonly injected into the subject""s blood stream for imaging the circulatory system or for imaging specific organs which absorb the injected radiopharmaceuticals. Gamma or scintillation camera detector heads are placed adjacent to a surface of the subject to monitor and record emitted radiation. For SPECT imaging, collimators are typically placed on the detector heads. For PET imaging, a coincidence detector detects concurrent receipt of a radiation event on two oppositely disposed heads. Often, the detector heads are rotated or indexed around the subject to monitor the emitted radiation from a plurality of directions. The monitored radiation data from the multiplicity of directions is reconstructed into a three-dimensional image representation of the radiopharmaceutical distribution within the subject. Such images typically provide functional and metabolic information.
Positron emission tomography (PET) is a branch of nuclear medicine in which a positron-emitting radiopharmaceutical such as 18F-fluorodeoxyglucose (FDG) is introduced into the body of a subject. Each emitted positron reacts with an electron in what is known as an annihilation event, thereby generating a pair of 511 keV gamma rays. The gamma rays are emitted in directions approximately 180xc2x0 apart, i.e., in opposite directions.
A pair of detectors registers the position and energy of the respective gamma rays, thereby providing information as to the position of the annihilation event and hence the positron source. Because the gamma rays travel in opposite directions, the positron annihilation is said to have occurred along a line of coincidence connecting the detected gamma rays. A number of such events are collected and used to reconstruct a clinically useful image.
The energy spectrum for clinical positron annihilation imaging is typically characterized by a photopeak at 511 keV. Similarly, Compton scattered radiation contributes to counts having energies ranging as high as the Compton edge. In coincidence imaging, a dual energy window detection scheme is sometimes used. A window around the photopeak and a window in the vicinity of the Compton region are identified. A coincidence event is counted if both detectors detect temporally simultaneous events within the photopeak window, or if one detector observes an event in the photopeak window while the other simultaneously detects an event in the Compton window. In each case, a memory location is incremented to note the event and its location such that the respective events are weighted equally. Events in which both detectors observe Compton events are discarded.
Single photon emission computed tomography (SPECT) is another nuclear imaging technique used to study the radionuclide distribution in subjects. Typically, one or more radiopharmaceuticals are injected into a subject. The radiopharmaceuticals are commonly injected into the subject""s blood stream for imaging the circulatory system or for imaging specific organs which absorb the injected radiopharmaceuticals. Gamma or scintillation camera heads are placed closely adjacent to a surface of the subject to monitor and record emitted radiation. In SPECT imaging, the detector head or heads are rotated or indexed around the subject to monitor the emitted radiation from a plurality of directions. The monitored radiation emission data from the multiplicity of directions is reconstructed into a three-dimensional image representation of the radiopharmaceutical distribution within the subject.
One of the problems with nuclear imaging techniques such as PET and SPECT is that photon absorption and scatter by portions of the subject between the emitting radionuclide and the camera head(s) distort the resultant image. One solution for compensating for photon attenuation is to assume uniform photon attenuation throughout the subject. That is, the subject is assumed to be completely homogenous in terms of radiation attenuation, with no distinction made for bone, soft tissue, lung, etc. This enables attenuation estimates to be made based on the surface contour of the subject. However, human subjects do not cause uniform radiation attenuation, especially in the chest.
In order to obtain more accurate radiation attenuation measurements, a direct measurement is made using transmission computed tomography techniques. In this technique, radiation is projected from a radiation source through the subject. Radiation that is not attenuated is received by detectors at the opposite side. The source and detectors are rotated to collect transmission data concurrently or sequentially with the emission data through a multiplicity of angles. This transmission data is reconstructed into a transmission image representation using conventional tomography algorithms. The radiation attenuation properties of the subject from the transmission image representation are used to correct for radiation attenuation in the emission data.
It is desirable to precisely locate the emission images relative to other anatomical details. In so doing, the diagnostic accuracy of the nuclear medicine image is increased. This is particularly so in the area of oncology, in which precise localization of nuclear medicine images aids in surgical and/or radiotherapeutic planning and for assessment of lesion progression and treatment effectiveness.
While transmission data has largely been successful in determining attenuation correction factors for the correction of the emission image data, the transmission image data itself has generally been of less than ideal resolution. The coarseness of the images could create uncertainties when localizing the emission image with respect to anatomical features.
One method of localizing the functional information is to merge the emission image representation with an image representation generated with another imaging modality that provides anatomical or structural details, such as x-ray computed tomography (CT), magnetic resonance (MR), or ultrasound image representations. When fusing images of different modalities, image registration of the two images is required to correct for any differences in geometric relations between the two images. Any misalignment of the two images impairs the diagnostic value of the fused images. Image registration can be performed by a number of techniques, such as using discrete extrinsic or intrinsic landmarks known to bear a constant relationship to the subject""s anatomy during the two studies, and using three-dimensional surface identification algorithms to construct numerical models of the external surface of the images. Such techniques allow the images to be aligned and oriented with respect to each other by translating, rotating, and descaling one or both of the image representations to allow the images to be superimposed or fused. However, given the lack of structural detail in nuclear medicine emission images and the low resolution of typical transmission images, significant uncertainties can remain when combining a nuclear medicine image with an image from a different modality. Moreover, inconvenience, cost, and multiple scans are generally required when obtaining scans from multiple modalities.
Imaging devices which combine a CT-like device with a gamma camera are known in the art. Such a device can reduce scan times by using correlated acquisition of nuclear medicine image data and CT image data. However, such a combined device is a less than optimal solution to the problem of nuclear medicine image localization due to cost and for logistical reasons. Also, although different modalities are combined on a single machine, this type of device retains the conventional approach of addressing separately the need for attenuation correction and the need for precise nuclear medicine image localization.
Transmission image quality can also be increased through increasing the number of counts, i.e., by increasing the source activity, increasing the imaging time, or both. Increasing the source activity, however, has the disadvantage of increasing cost and shielding requirements. Increasing the imaging time is generally undesirable for patient handling reasons. Also, both increasing source activity and increasing imaging time undesirably increase the dose of radiation received by the subject.
Accordingly, the present invention contemplates a new and improved nuclear medicine imaging method and apparatus which overcome the above-referenced problems and others.
In accordance with a first aspect of the present invention, a method of generating an image comprises providing a transmission radiation source emitting gamma rays at a plurality of energy levels and directing the emitted gamma rays through a subject to be imaged, the subject attenuating the transmission of the gamma rays and defining an energy range encompassing the emission energy levels. The energy range is divided into a plurality of energy subranges and gamma rays passing through the subject and falling within the defined energy range are detected. Detector head positions or trajectories and energies of the detected gamma rays are determined and this information is logged into a plurality of image data subsets based on the determined energy of the detected gamma rays, wherein each image data subset corresponds to one of the energy subranges. The steps of detecting, determining, and logging are repeated for a plurality of transmitted rays. The image data subsets are compared to determine one or both of: (1) variations in attenuation between different tissue types of the subject as a function of energy, and (2) variations in attenuation within each tissue type of the subject as a function of energy. Based on the determined attenuation variations, a weighting factor is assigned to each of the image data subsets and the image data subsets are combined in accordance with their assigned weighting factors to produce a weighted image data set, the weighting factors being assigned so as to enhance at least one structural feature in the weighted image data set. A feature-enhanced transmission image representation representative of the weighted image data set is then generated.
According to another aspect, a method of diagnostic imaging comprises transmitting radiation with a defined energy spectrum through a subject and converting the transmission radiation which has traversed the subject into electronic transmission data indicative of transmission radiation detector head position or trajectory and energy. In accordance with the energy data, the transmission trajectory data is sorted into a plurality of energy windows and the transmission trajectory data in each window is reconstructed into a corresponding electronic transmission image representation. The electronic transmission image representations are each weighted and the weighted transmission image representations are combined.
In yet a further aspect, the present invention provides a gamma camera comprising a transmission radiation source for generating radiation im a selected energy range and a detector for detecting emission radiation emitted from within a subject and transmission radiation from the transmission radiation source which has traversed a subject to be imaged, the subject attenuating the radiation, the detector generating position or trajectory and energy data. The gamma camera further comprises energy discrimination circuitry connected with the detector, the energy discrimination circuitry sorting detected transmission radiation in accordance with a plurality of energy subranges within the selected energy range; an electronic storage medium connected with the energy discrimination circuitry, the electronic storage medium storing a plurality of transmission data subsets, the data subsets comprising data grouped by energy in accordance with the plurality of energy subranges; at least one reconstruction processor connected with the electronic storage medium which generates a transmission image representation for each of the plurality data subsets; and a combine processor connected with the reconstruction processor which weights the transmission image representations and combines the plurality of weighted image representations to produce at least one weighted image representation, the weighting being selected to enhance at least one selected feature in each weighted image representation.
One advantage of the present invention is that it provides a transmission image providing increased anatomic structural detail without increasing imaging time or increasing radiation source activity.
Another advantage is that the transmission image data is maintained in a proper format for attenuation correction of the emission image data.
Another advantage of the present invention is that it provides enhanced transmission image data for improved registration of an emission image representation with an image representation of a different modality.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.